The Trouble with High Burn-Up Nuclear Fuel

Spent nuclear fuel is hot stuff. It’s thermally hot — about 400 degrees Fahrenheit. That’s not residual heat from when the fuel was in the reactor, it’s decay heat from fission products with relatively short half-lives – from days or weeks to about 30 years for most of them (most isotopes of iodine, cesium, strontium, etc.). The fuel will stay well above the boiling point of water for centuries or even millennia, although the temperature will keep dropping over time.

(Note: The term “short” for the half-lives of most fission products compares to Uranium, which is a billion years or more, or even Plutonium, which is 10s of thousands of times more radioactive (SHORTER half-life) than Uranium. Fission products are thousands of times more deadly than that, not counting Pu and U’s heavy metal horrors.)

And speaking of the boiling point of water, above that you get steam. Steam is particularly hazardous to the zirconium cladding of the fuel rods. The zirconium separates the hydrogen from the oxygen in the water molecules of the steam, and the hydrogen atoms combine with each other as H2, which is explosive. Because it’s so hot and radioactive inside the dry cask, they can’t monitor this process near where it’s happening, inside the “dry” cask. They need to monitor the water content, as well as the hydrogen, oxygen, helium, and “fission gasses” that are emitted.

After draining the fuel rods by slowly lifting the entire dry cask assembly out of the spent fuel pool (about 15 years after it was used in the reactor) about 25 gallons of water will remain in the fuel assembly. This water must be removed through repeated drying processes which are only partially successful each time. After that, water seepage into the dry cask is also an ever-constant threat.
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